Higher operating temperatures of gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys, though such alloys alone are often inadequate to form components located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to thermally insulate such components in order to minimize their service temperatures. For this purpose, thermal barrier coatings (TBC) formed on the exposed surfaces of high temperature components have found wide use.
To be effective, thermal barrier coatings must have low thermal conductivity, capable of being strongly adhered to the article, and remain adherent through many heating and cooling cycles. The latter requirement is particular demanding due to the different coefficients of thermal expansion between materials having low thermal conductivity and superalloy materials used to form turbine engine components. For this reason, thermal barrier coatings have generally employed a metallic bond layer deposited on the surface of a superalloy component, followed by an adherent ceramic layer that serves to thermally insulate the component, which together form what is termed a thermal barrier coating system. The metallic bond layer is formed from an oxidation-resistant alloy, such as MCrAlY where M is iron, cobalt and/or nickel, and promotes the adhesion of the ceramic layer to the component while also preventing oxidation of the underlying superalloy. Various ceramic materials have been employed as the ceramic layer, particularly zirconia (ZrO.sub.2) stabilized by yttria (Y.sub.2 O.sub.3), magnesia (MgO) or another oxide. These particular materials are widely employed in the art because they can be readily deposited by plasma spray, flame spray and vapor deposition techniques, and are reflective to infrared radiation so as to minimize the absorption of radiated heat by the coated component.
A continuing challenge of thermal barrier coating systems has been the formation of a more adherent ceramic layer that is less susceptible to spalling when subjected to thermal cycling. For this purpose, the prior art has proposed various coating systems, with considerable emphasis on ceramic layers having enhanced strain tolerance as a result of the presence of porosity, microcracks and segmentation of the ceramic layer. Microcracks generally denote random internal discontinuities within the ceramic layer, while segmentation indicates that the ceramic layer has a columnar grain structure oriented perpendicularly to the surface of the component, such that microcracks are present between grains. As is known in the art, a thermal barrier coating having a columnar grain structure is able to expand without causing damaging stresses that lead to spallation, as evidenced by thermal cyclic testing.
Zirconia-base thermal barrier coatings, and particularly yttria-stabilized zirconia (YSZ) coatings having columnar grain structures and a thickness on the order of about 125 micrometers (about 0.005 inch) or more, are widely employed in the art for their desirable thermal and adhesion characteristics. However, YSZ coatings are susceptible to erosion from particles and debris present in the high velocity gas stream of a gas turbine engine. Though a solution to a high erosion rate is to increase the thickness of a thermal barrier coating, a drawback is an increase in thermal stresses within the coating, leading to a higher incidence of spallation. Consequently, there is a need for erosion-resistant thermal barrier coating systems having minimal thicknesses, preferably less than 125 micrometers.
Attempts to produce erosion-resistant thermal barrier coating systems for high temperature applications in a gas turbine engine have been directed to thermally treating the outer surface of the ceramic material or providing an additional wear-resistant outer coating to promote the erosion resistance of the coating system. More wear-resistant outer coating materials suggested in the past have included zircon (ZrSiO.sub.4), silica (SiO.sub.2), chromia (Cr.sub.2 O.sub.3) and alumina (Al.sub.2 O.sub.3). While various methods and apparatuses have been suggested that are capable of depositing layers of different materials, as evidenced by U.S. Pat. No. 3,205,087 to Allen and U.S. Pat. No. 4,632,059 to Flatscher et al., difficulty has been encountered when attempting to sequentially deposit discrete and homogeneous layers. In particular, compositional gradients tend to form between successively deposited layers, which generally increases the thermal conductivity of the thermal barrier coating.
Attempts have also been made using multilayer systems (microlaminates) to reduce the thermal conductivity of thermal barrier coatings in order to allow use of thinner coatings. Approaches have included YSZ and alumina as the alternating materials, such that the interfaces between layers help to inhibit the transfer of heat. As noted above, prior art processes have not reliably achieved the sharp compositional gradients required to have the desired effect of lowering thermal conductivity.
The desire to reduce thermal stresses by minimizing the thickness of the overall thermal barrier coating further complicates the process by which such thermal barrier coatings are formed. Though it is possible to employ low deposition rates, as taught by Allen and inherent with the teachings of Flatscher et al., and physically reposition a component to be coated in order to ensure that discrete and homogeneous layers are deposited, such a technique is impractical for coating mass-produced components such as turbine blades and vanes of a gas turbine engine.
Accordingly, what is needed is a method for forming a multilayer thermal barrier coating system characterized by lower thermal conductivity and enhanced resistance to spallation when subjected to erosion in a hostile thermal environment, in which the method is particularly well suited for use in mass production.